1. Field of the Invention
The invention relates to contact probes for use in testing electrical circuits, particularly electrical circuits on printed circuit boards or printed wire boards. More particularly, the invention relates to an electrical contact probe comprising a holder holding one or more fibers having low electrical resistance and high mechanical strength for use in testing electrical circuits. The invention also relates to an electrical test device that uses the contact probe.
2. Description of the Related Art
The integrity of semiconductor device elements such as printed circuit boards and printed wire boards are generally evaluated shortly after manufacturing in order to identify and eliminate various faults such as short circuits or open circuits. Quality testing of this kind reduces the number of defective elements and leads to the highly reliable and maintenance-free electrical devices that are used today.
Two general types of tests are currently used to evaluate the electrical connections in newly manufactured semiconductor elements. In one type of test, the element is mounted on a mounting stand and measurement pads are brought into electrical contact with a card having probe pins arranged in a one- or two-dimensional array specific for the particular semiconductor element being tested. An electrical signal is sent through the card and monitored for open or short circuits. Elements that pass these tests are packaged and incorporated into other devices. Defective elements are either repaired or discarded.
However, this test approach suffers several disadvantages. First, the array of pins necessary to test the circuit are specific to a single circuit pattern. Accordingly, these test cards are time-consuming and expensive to produce and have limited usefulness. Also, as the circuit spacing in integrated electrical circuits is reduced, this method of testing becomes increasingly difficult to implement successfully due to spacing limitations between the pins. A lower circuit spacing limit for this testing approach is approximately 0.025 inches. Circuits having circuit spacings below this limit are generally not amenable to this type of testing procedure.
A second testing approach utilizes one or more moveable, stiff, needle-like probes, typically made from a conductive metal such as copper, tungsten or titanium. A typical test cycle using this approach includes (1) raising the probe from the surface of the element being tested, (2) moving the probe to a position over the next circuit to be tested, and (3) lowering the probe to make electrical contact with the circuit.
This approach also suffers several disadvantages. First, this method of testing circuits requires stiff probes that must slow down and stop above selected contact test pads before being moved downward until contact is made for each continuity determination. This sequence of moves, starts, and stops must be repeated for every continuity determination (which can be many thousands of times for each circuit board). This tedious procedure results in the probes spending a significant percentage of the total testing time executing moves, starts and stops. In addition, the probe must be moved in three dimensions, thus requiring a highly complex moving apparatus. Also, in order for the testing to take place, the stiff probe must come into contact with the circuit pads (test points). Frequently, significant mechanical force is required to establish contact between the probe and the circuit pad, and can lead to damage of the circuit pad or the probe if the degree of applied force is not monitored carefully.
The following U.S. Patents represent the general state of the art:
U.S. Pat. No. 5,218,757 issued to Kaneko et al. a method of making tapered carbon microelectrodes for use in various electrochemical measurements and scanning tunneling microscopes. However, the disclosed microelectrode made from the process is coated with an insulative material and is not used for testing electrical circuitry.
U.S. Pat. No. 5,532,613 to Nagasawa et al. discloses an electrical probe needle that is coated with a conductive film and shielded with an insulating film. The insulation applied to each probe needle alleviates noise and mutual crosstalk between signal currents. Additionally, the insulative layer applied to each probe needle prevents short-circuiting if the needles touch each other.
U.S. Pat. No. 5,596,283 to Mellitz et al. discloses an electrical test method and apparatus that includes a probe having a rolling ball at the tip. However, tips of this type are generally made from conductive metals and not organic (e.g., carbon) materials.
Thus, the current state of the art methods for testing electrical circuits requires stiff, nonfiber probes that must slow down and stop above selected pads before being moved downward until contact is made for each continuity determination. In addition, current testing procedures require that the probe and contact pad are subjected to significant mechanical force which can result in surface damage. What is needed in the art is a precision probe with a high spatial resolution that can be operated at relatively high speed without physically affecting the circuit test pads. The present invention is believed to be an answer to that need.